Method of repairing a ceramic coating

ABSTRACT

It is a method of repairing a ceramic coating of an article after use of this article in a high temperature environment disclosed. The ceramic coating is removed locally at spalled areas, at the areas where the ceramic coating is removed locally a mixture including a powder of zirconia stabilized with one or a combination of yttria, calcia, scandia, magnesia, ceria and oxides of the rare earth group, and hydrated metallic halides as a binder is applied and the applied ceramic powder is dried.

FIELD OF INVENTION

[0001] The invention relates to a process of repairing a ceramic coatingof a component according to the preamble of claim 1.

STATE OF THE ART

[0002] Components designed for the use in the area of high temperature,e.g. blades or vanes of a gas turbine, are usually coated withenvironmentally resistant coatings. The coating protects the basematerial against corrosion and oxidation due to the thermal effect ofthe hot environment and consists of an alloy mostly using the elementsAl and Cr. Most turbine components are coated for protection fromoxidation and/or corrosion with, for example, a MCrAlY coating (basecoat) and some are also coated with a thermal barrier coating (TBC) forthermal insulation. MCrAlY protective overlay coatings are widely knownin the prior art. They are a family of high temperature coatings,wherein M is selected from one or a combination of iron, nickel andcobalt. As an example U.S. Pat. No. 3,528,861 or U.S. Pat. No. 4,585,418disclose such kind of oxidation resistant coatings. U.S. Pat. No.4,152,223 as well discloses such method of coating and the coatingitself.

[0003] Furthermore, in the state of the art Thermal-Barrier-Coatings(TBC) are known from different patents. U.S. Pat. No. 4,055,705, U.S.Pat. No. 4,248,904, U.S. Pat. No. 4,321,311 or U.S. Pat. No. 4,676,994disclose a TBC-coating for the use in the turbine blades and vanes. Theceramics used are yttria stabilized zirconia and applied by plasma spray(U.S. Pat. No. 4,055,705, U.S. Pat. No. 4,248,940) or by electron beamprocess (U.S. Pat. No. 4,321,311, U.S. Pat. No. 4,676,994) on top of theMCrAlY bond coat.

[0004] The demands of operation of the parts in a gas turbine often leadto the degradation of the coating before the structural integrity of theunderlying part itself is degraded. Hence, the base coat and TBC must beremoved and reapplied. Such processes are known from EP-A2-813 930,EP-A1-298 309, U.S. Pat. No. 6,158,957, U.S. Pat. No. 6,132,520, U.S.Pat. No. 6,146,692, U.S. Pat. No. 5,728,227, U.S. Pat. No. 5,614,054 andU.S. Pat. No. 5,900,102. Many of these inventions such as U.S. Pat. No.5,972,424, U.S. Pat. No. 6,158,957, U.S. Pat. No. 6,132,520 and U.S.Pat. No. 6,146,692 deal with the removal of TBC by caustic treatment inan autoclave at modest to elevated temperatures and pressures. A secondmethod often used is the gaseous fluoride treatment such as gaseoushydrogen fluoride as disclosed in U.S. Pat. No. 5,728,227 or U.S. Pat.No. 5,614,054. Yet in another method, U.S. Pat. No. 5,900,102, the TBCwas removed by treatment with a mixture of powders consisting of ahalogen containing activator (i.e. ammonium fluoride, chloride etc),aluminum metal and inert alumina as a filler, again the reaction productis a gaseous fluoride, as in the above citations. It is to be noted thatalthough the above inventions teach us only the methods of removing thedamaged TBC from a component, none of them provide any new or noblemethod or a process for re-coat of the component from where TBC wasremoved—simply the TBC in the prior art was reapplied by theconventional plasma spray method.

[0005] However, in the prior art U.S. Pat. No. 5,759,932 and U.S. Pat.No. 5,985,368 disclose methods wherein the TBC is applied on the surfaceusing a slurry. In essence the powdered yttria stabilized zirconiawith/without porous zirconia spheres is blended in a paint mixer with asilicone resin, SR-350 and a lubricant, UVCON-50MB-2000.

[0006] It is also known that turbine blades and other components haveonly local areas of extremely high surface temperature during operation,whereas the rest of the component surface has a moderate temperature.This means that the environmentally protective coatings are degraded byfar the most in these local areas of high temperature, which constituteabout 5 to 20% of the total surface area of the whole component, andonly moderately over the rest of the surface area.

SUMMARY OF THE INVENTION

[0007] It is object of the present invention to find a method of locallyrepairing thermal barrier coated components. This method should beespecially suitable for the local repair of turbine blades and vanes ofboth aircraft and industrial gas turbine engines. Another objective isto prove a method of repairing a TBC on a site in the field.

[0008] According to the invention a method was found of repairing anarticle with a ceramic coating according to the preamble of the claim 1wherein

[0009] a. remnants of the ceramic coating adjacent to and inside of theareas of spallation are removed,

[0010] b. the areas of spallation are prepared for local coatingapplication by conventional means such as chemical treatment, gritblasting or abrasive cleaning,

[0011] c. on the areas of spallation is applied a mixture including apowder of zirconia stabilized with one or a combination of yttria,calcia, scandia, magnesia, ceria and oxides of the rare earth group, andhydrated metallic halides as a binder, after which

[0012] d. the applied mixture of ceramic powder and binder is dried.

[0013] With advantage the mixture of stabilized zirconia powder andhydrated metallic halides as a binder is prepared as a slurry with wateralone or with water in combination with alcohol using hydrated aluminumchlorides before applying to the mentioned areas. The viscosity of theslurry can be adjusted by the addition of distilled water alone or bythe addition of water in combination with alcohol to obtain a suitableflowability during spraying. In some cases the viscosity of the hydratedmetallic hydride will be sufficient that no water or alcohol need beadded to the slurry to achieve the desired properties. Hollow zirconiaspheres could be added to the mixture of stabilized zirconia powder andhydrated metallic hydrides in order to offer the advantage of a highereffective porosity content of the TBC.

[0014] In one embodiment, the slurry coating is dried at a temperaturebetween 20° C. and 100° C. and finally heat-treated at 55° to 750° C. toremove the evolving halogens, halogen gas, or residual water.

[0015] This method saves both time and investment costs becausestripping and re-coating the entire surface area of the component usingconventional means such as plasma spraying is unnecessary.

[0016] The hydrated metallic halide binder can consist of one or acombination of hydrated metallic halide components such as hydratedaluminum chloride and/or hydrated halides of magnesium, calcium,yttrium, zirconium, strontium, barium, including other hydrated halidesof the rare earth group or group 11 alkaline halides.

[0017] In another embodiment of the invention, the sizes of individualpowder particles of the stabilized zirconia powder range from 5 to 150μm to offer the advantages of greater strength and lifetime on thecomponent during service.

[0018] This invention is particularly useful when applied to articlessuch as blades, vanes or any other gas turbine component operating athigh temperatures and coated with a TBC.

DETAILED DESCRIPTION OF INVENTION

[0019] The invention is related to a process of repairing an articlewith a TBC-coating being exposed to a high temperature environment ordamaged due to the transport after the manufacture. The damage receivedby the component consists of spallation of the TBC localized to theareas of highest temperatures experienced during service or areas whichwere impacted due to Foreign Object Damage during service in the gasturbine. For the case of damage during shipping or other transport, theareas affected are localized to areas of impaction of the TBC on anotherhard object due to improper handling or packing. The article couldpossibly be a blade or a vane or any other part of a gas turbine enginesuch as a part of a burner chamber exposed to the hot gases of the gasturbine, the article being coated with a MCrAlY-coating to protect itagainst oxidation. On top of the MCrAlY there is a Thermal BarrierCoating known e.g. from different patents U.S. Pat. No. 4,055,705, U.S.Pat. No. 4,248,904, U.S. Pat. No. 4,321,311 or U.S. Pat. No. 4,676,994.

[0020] As base material often used is a superalloy e.g. Ni or Co based.A Ni based superalloy is disclosed in the patent U.S. Pat. No.5,759,301.

[0021] It is well known to those skilled in the art that the types oflocalized damage described above have little or no effect on thelifetime of the TBC on the rest of the component. However, the TBCimmediately surrounding the areas of spallation is often weakened andmust be removed, and additionally there are often small residual amountsof TBC left on the area of spallation. Therefore according to theinvention all the weakened TBC material adjacent to the area ofspallation must be removed in order to insure a high quality repair, andany residual TBC within the general area of spallation must be removedto ensure proper bonding of the TBC repair material. The ceramic TBCcoating could be removed locally by any conventional abrasive means suchas grit blasting or grinding, and/or by a chemical stripping such aschemical etching using an acid and/or a mixture of acids. Any residualoxidized MCrAlY material from the bondcoat should also be removed by theabove means to provide a high quality surface for the repair material tobe applied.

[0022] At the areas where the ceramic coating is removed locally, amixture including a powder of zirconia stabilized with one or acombination of yttria, calcia, scandia, magnesia, ceria and oxides ofthe rare earth group, and hydrated metallic halides as a binder isapplied and the applied mixture is dried at moderate temperatures,whereby the constituents are tenaciously bound together. In oneembodiment, the slurry coating is dried at a temperature of 20° to 100°C. and is then finally heat-treated at 55-750° C. to remove the evolvinghalogen compounds, halogen gases or residual water and/or alcohol.

[0023] The mixture of stabilized zirconia powder and hydrated metallichalides may offer the desired consistency, viscosity, bondingproperties, and drying properties without any further addition. However,in some cases it is advantageous to make additions of distilled waterand/or alcohol to the mixture in order to further control the viscosityof the slurry and obtain a suitable flowability during application.

[0024] The hydrated metallic halide binder can consist of one or acombination of hydrated metallic halide components such as hydratedaluminum chloride and/or hydrated halides of magnesium, calcium,yttrium, zirconium, strontium, barium, including other hydrated halidesof the rare earth group or alkaline halides (group of the period systemof the elements). The hydrated metallic hydrides are particularlyadvantageous to use as binders in this application. It is known to thoseskilled in the art that, e.g. hydrated aluminum chloride when exposed toelevated temperatures will decompose to hydrogen choride and water,which leave as gases, and alumina which bonds the yttria stabilizedzirconia powder to itself and to the substrate and provides alphaalumina in the TBC known to provide oxidation resistance.

[0025] In an advantageous embodiment, the sizes of individual powderparticles of the stabilized zirconia powder ranges from 5 to 150 μm tooffer the advantages of greater strength and lifetime on the componentduring service.

[0026] The mixture of stabilized zirconia powder and hydrated metallichydrides can be applied by painting, slurry spraying, or as a paste.Furthermore, in some cases it is advantageous to apply an initial layerof highly fluid slurry in order to maximize wetting of the local area tobe repaired and provide good bonding of the next layer of repairmaterial to the surface, and then apply a more viscous version of theslurry in order to ensure optimal structure during drying.

[0027] The method according to the present invention saves both time andinvestments costs because stripping and re-coating the entire TBC-coatedportion of the component using conventional means such as plasmaspraying is unnecessary. A further advantage is that the repair processis rapid, and if the appropriate equipment is used, can be carried outin the field with no need for detachment of the component from theengine assembly, nor any need for sending the component to a repairshop.

[0028] The invention is particularly advantageous when applied to ablade or a vane or any other gas turbine component consisting of anickel or cobalt base alloy exposed to a high temperature environmentand coated with TBC.

1. A method of repairing a ceramic coating of an article after damagedue to the transport after manufacture or use of this article in a hightemperature environment, wherein a. remnants of the ceramic coatingadjacent to and inside of the areas of spallation are removed, b. theareas of spallation are prepared for local coating application byconventional means such as chemical treatment, grit blasting or abrasivecleaning, c. on the areas of spallation is applied a mixture including apowder of zirconia stabilized with one or a combination of yttria,calcia, scandia, magnesia, ceria and oxides of the rare earth group, andhydrated metallic halides as a binder, after which d. the appliedmixture of ceramic powder and binder is dried.
 2. The method of claim 1,wherein the mixture of stabilized zirconia powder and hydrated metallichalides as a binder is prepared as a slurry with water alone or withwater in combination with alcohol using hydrated aluminum chloridesbefore applying to the mentioned areas.
 3. The method of claim 2,wherein the slurry is dried at a temperature between 20° C. and 100° C.and finally heat-treated at 55° to 750° C.
 4. The method of claim 2,wherein hollow zirconia spheres are included in the mixture ofstabilized zirconia powder and hydrated metallic hydrides.
 5. The methodof claim 1, wherein the hydrated metallic halide binder consists of oneor a combination of hydrated metallic halide components such as hydratedaluminum chloride and/or hydrated halides of magnesium, calcium,yttrium, zirconium, strontium, barium, including other hydrated halidesof the rare earth group or group 11 alkaline halides.
 6. The method ofclaim 1, wherein the sizes of individual powder particles of thestabilized zirconia powder ranges from 5 to 150 μm.
 7. The method ofclaim 1, wherein the mixture of stabilized zirconia powder and hydratedmetallic hydrides is applied by painting, slurry spraying, or as apaste.
 8. The method of claim 1, wherein the article is a blade or avane or any other part of a gas turbine engine consisting of a nickel orcobalt base alloy exposed to a high temperature environment coated withTBC.